The prior art teaches equipping vehicles with “variable displacement,” “displacement on demand,” or “multiple displacement” internal combustion engines in which one or more cylinders may be selectively “deactivated” or “suppressed,” for example, to improve vehicle fuel economy when operating under relatively low-load conditions. Typically, the cylinders are deactivated through use of deactivatable valve train components, such as deactivating valve lifters as disclosed in U.S. patent publication no. US 2004/0244751 A1, in which a supply of pressurized engine oil is selectively delivered from an engine oil gallery to a deactivatable valve lifter through operation of a solenoid valve under the control of an engine control module. Preferably, the engine control module operates the solenoid valve such that the lifter's locking pins are moved between their respective locked and unlocked positions as the lifter's cam lies on the base circle of its corresponding cam surface, thereby minimizing lifter wear and noise. Thus, the triggering of the oil control solenoids is preferably synchronized to either the crankshaft in a pushrod engine, or the cam shaft in an overhead cam engine.
It is also known that, at each engine speed, there is a range of potential solenoid trigger points that produce a proper sequencing of the deactivatable valve train components, with the deactivation triggering window being significantly “wider” than the reactivation window because less time is needed to increase the oil gallery pressure to the relatively-lower unlatching pressure, as opposed to dropping the oil gallery pressure from a relatively-higher sustained pressure down to the latching pressure. It is further known that the viscosity of the oil supplied to the oil gallery has a significant impact on the amount of time required for deactivation, as a more viscous oil will drain more slowly through the solenoid's and/or the actuator's drain passages, so the prior art has sought to delay the enablement of actuator operation until the engine oil is sure to be warm, for example, by enabling actuator operation only after a minimum engine run time has occurred, or a minimum engine coolant temperature has been achieved.
However, such prior art approaches necessarily prevent early utilization of the deactivatable system, for example, to improve vehicle fuel economy, if the instantaneous oil viscosity is otherwise suitable for actuator operation before the timer has run out, or before the minimum engine coolant temperature has been achieved. Further, such prior art approaches do not accommodate changes in the nominal viscosity of the engine's oil over time, as through aging/oil breakdown (shear) or, perhaps, through operator error as may occur when refilling/replacing engine oil with an oil having a different nominal viscosity rating or “grade.” It is, therefore, desirable to correlate enablement of an actuator's operation with a measure representing the instantaneous nominal viscosity of the supplied engine oil.